As the use of lithium secondary batteries has been gradually expanded, demand for a large size and high capacity has become increased. Further, in recent years, the capacity of the battery has been rapidly increasing, and an area of an electrode plate used for this purpose is also rapidly widening. In the case of such a high capacity battery, when the battery is charged and discharged for a long period of time, a cathode electrode plate and an anode electrode plate may not be in close contact with each other but separated, or the battery may be expanded or deformed, and thus, a lifetime of the battery may be reduced.
Accordingly, various methods for imparting functionality to a porous thin film type film formed of polyethylene or polypropylene, which is a separation membrane used in the lithium secondary battery, have been attempted. As one of such attempts, there is a method for improving performance of a battery by coating fusion layers on both sides of the separation membrane, and fusing the separation membrane with a cathode and an anode of the battery. That is, it is possible to improve a lifetime of the battery by imparting the separation membrane to a fusion property (or adhesive property) and fusing the separation membrane with electrodes to prevent separation of electrode plates or expansion or deformation of the battery as described above.
Meanwhile, when heat stability of a polyolefin-based microporous membrane is reduced, a short-circuit between the electrodes accompanied with damage or deformation of the microporous membrane may occur due to temperature rise caused by abnormal behavior of the battery. Further, there is a risk of overheating, ignition or explosion of the battery.
Thus, an approach to improve safety of the battery by imparting a separate property in addition to the above-described fusion property to the separation membrane, is in progress.
Japanese Patent No. 4,127,989 discloses a separation membrane in which a porous fusion layer formed of an organic polymer that swells an electrolyte on both sides of a base of a polyolefin microporous membrane and supports the electrolyte is disposed. The above technique has advantages in view of ion conductivity and fusion property, but has a problem in that thermal resistance is insufficient, and the fusion layer has a thick thickness, which is contrary to the demand for thinning the separation membrane.
Korean Patent No. 1156961 discloses that by partially coating a silane-based compound on upper and lower outer circumferential surfaces of the electrode and the separation membrane, it is possible to prevent deterioration of the ion conductivity by a fusion layer while increasing fusion force. However, a large-area battery has a limitation in that when only the vicinity of the upper and lower outer circumferential surfaces of the separation membrane is fused, a middle portion of the electrode may not be prevented from being separated.